Ultrashort-pulse source with controllable multiple-wavelength output

ABSTRACT

A multiple-wavelength ultrashort-pulse laser system includes a laser generator producing ultrashort pulses at a fixed wavelength, and at least one and preferably a plurality of wavelength conversion channels. Preferably, a fiber laser system is used for generating single-wavelength, ultrashort pulses. An optical split switch matrix directs the pulses from the laser generator into at least one of the wavelength conversion channels. An optical combining switch matrix is disposed downstream of the wavelength-conversion channels and combines outputs from separate wavelength-conversion channels into a single output channel. Preferably, waveguides formed in a ferroelectric substrate by titanium indiffusion (TI) and/or proton exchange (PE) form the wavelength conversion channels and the splitting and combining matrices. Use of the waveguide allows efficient optical parametric generation to occur in the wavelength-conversion channels at pulse energies achievable with a mode-locked laser source. The multiple-wavelength laser system can replace a plurality of different, single-wavelength laser systems. One particular application for the system is a multi-photon microscope, where the ability to select the ultrashort-signal wavelength of the laser source accommodates any single fluorescent dye or several fluorescent dyes simultaneously. In its simplest form, the system can be used to convert the laser wavelength to a more favorable wavelength For example, pulses generated at 1.55 μm by a mode-locked erbium fiber laser can be converted to 1.3 μm for use in optical coherence tomography or to 1.04-1.12 μm for amplification by a Yterbium amplifier, allowing amplification of pulses which can be used in a display, printing or machining system.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to an apparatus andmethod for generating ultrashort optical pulses at a plurality ofoptical wavelength and, more particularly, to an apparatus and methodusing optical fibers and optical waveguides to produce and control suchoptical pulses. Ultrashort is here generally referred to as being withinthe time scale of approximately 10⁻¹⁵ seconds (ferntoseconds) to 10⁻¹²seconds (picoseconds). The present invention further relates to a methodand apparatus for optical imaging using ultrashort optical pulsessimultaneously emitted at a plurality of optical wavelengths.

[0003] 2. Description of the Related Art

[0004] A variety of laser systems for producing ultrashort opticalpulses is known in the prior art. From a practical point of view, theseSystems can be generally grouped into two main categories: solid-statelaser systems which are based on the use of volume laser gain media, andfiber laser systems, which arm based on waveguiding fiber-opticcomponents. Due to their intrinsic structure, fiber lasers have a numberof basic properties which make them significantly more suitable forwidespread practical use. As is well known in the prior art, fiberlasers are compact, can be diode pumped, and arc robust and reliable,For a number of reasons, at present, the most mature technology suitablefor ultrashort-pulse fiber laser systems is based on Er-doped fiberproviding output pulses having a wavelength of approximately 1.55 μm.Fist, Er-doped fibers are among the best developed of therare-earth-doped fibers. Diode lasers for pumping such fibers are alsowell advanced.

[0005] Significantly, the generation of ultrashort pulses requiresdesign-control of the dispersion in the laser cavity. This can beaccomplished in a compact, all-fiber cavity only at wavelengths above1.3 μm. where the dispersion of the optical fiber can be tailored to beeither of positive or negative sign. However, a variety of practicalapplications for ultrashort pulses require other wavelengths ofoperation, for example, either at shorter or longer wavelengths. Atthose wavelengths, ferntosecond-pulse fiber oscillators at present canbe designed only by using bulky external components, such as sets ofprism pairs, to control the in-cavity dispersion.

[0006] For many applications, the wavelength of the laser is critical.For example, for confocal microscopy used in cellular biology, specificdyes are attached to different parts of the cell and are used to observedifferent functions. Each of these dyes is excited to fluorescence by arespective spectrum of light Thus, for confocal microscopy, a pluralityof different wavelength lasers are used for different dyes. Recently,ultrafast lasers with short pulses and high peak power have been used toexcite dyes at resonances which require two-photon excitation. That is,ultrafast lasers have been used to supply enough photon density at thefocus of a microscope to cause a non-linear optical effect, called thetwo-photon absorption effect. This effect is used to excite dyes at theenergy level which corresponds to half of the wavelength of each of thetwo original photons. However, the number of lasers available atdifferent wavelengths is limited; consequently there are only a few dyeswhich can be utilized at this time. Therefore, the field of two photonmicroscopy could benefit greatly from a laser capable of being widelytuned to the different wavelengths corresponding to a number ofdifferent dyes. The current accepted specifications for a laser forscanning two-photon microscopy are 10-30 mw average power, 100-200 fspulse width and 50-100 M repetition rate, The general and well knownmethod to extend the wavelength range of any particular laser system isto utilize nonlinear optical interactions, such as optical harmonicgeneration, sum or difference frequency generation and opticalparametric gain.

[0007] Harmonic generation is suitable only for converting an opticalsignal to a higher optical frequency (shorter wavelength) and it cannotprovide able or multiple-wavelength output. Sum-frequency anddifference-frequency generation allows conversion of a signal to bothhigher and lower optical frequencies and allows wavelength tunability,but requires at least two well synchronized optical sources at twodifferent optical frequencies. Therefore, each of these interactionsalone cannot provide multiple-wavelength or wavelength-tunable outputfrom one, single-wavelength signal source.

[0008] Optical parametric interaction is suitable for providing tunableor multiple-wavelength conversion using one, single-wavelength opticalsignal source. Furthermore, while optical parametric conversion allowsconversion of an optical signal only to a lower optical frequency(longer wavelength), by combining parametric interaction with at leastone of the above described interactions, any optical frequency above orbelow the signal-source frequency can be obtained.

[0009] The general drawback of parametric optical frequency conversionis that, in order to achieve high parametric gain sufficient to amplifyspontaneous quantum-fluctuation noise from microscopic to macroscopiclevels and, consequently, to achieve efficient signal-energy conversion,high peak-powers and high pulse-energies are required It is well knownfrom the prior art that the required energies are well above theenergies that can be generated directly from a typical mode-locked,ultrashort-pulse laser oscillator. The best demonstrated result known todate is an optical parametric generation (OPG) threshold at ˜50 nJ, andefficient OPG conversion of ˜40% at approximately 100 nj achieved inbulk periodically-poled lithium-niobate crystals, as reported byGalvanauskas et al. in “Fiber-laser-based ferntosecond-parametricgenerator in bulk periodically poled LnbO ₃”; Optics Letters, Vol. 22,No. 2 January 1997. In comparison, typical ferntosecond mode-lockedpulse energies from a fiber laser are in the range of 10 pJ to 10 nJ (asdescribed by Fermann et al. in “Environmentally stable Kerr-typemode-locked erbium fiber laser producing 360-fs pulses”; Optics Letters;Vol. 19, No. 1; January, 1997, and by Fermann et al. in “Generation of10 nJ picosecond pulses from a mode-locked fibre laser”; ElectronicsLetters Vol. 31, No. 3; February, 1995) and those from a solid-statelaser are in the range of up to ˜30 nJ (as described by Pelouch et al.in “Ti:sapphire-pumped, high-repetition-rate ferntosecond opticalparametric oscillator”; Optics Letters, Vol. 17, No. 15; August, 1992).

[0010] It is known from the prior art that efficient optical parametricwavelength conversion can be achieved with unamplified or amplifiedmode-locked laser pulses by arranging a nonlinear crystal in a separateoptical cavity in a manner that ensures that pump pulses and signalpulses pass the parametric gain medium synchronously, as seen, forexample in the above-referenced article by Pelouch et al. Since, in thiscase, parametric interaction occurs repetitively, the low, single-passparametric gain and, consequently, low pulse energies of mode-lockedoscillators are sufficient to achieve efficient conversion. Thesignificant practical drawback of this approach is that such a schemerequires two precisely length-matched optical cavities; one for amode-locked oscillator and another a for synchronously-pumped opticalparametric oscillator (OPO). Consequently, such OPO systems are complex,large, and intrinsically very sensitive to the environmental conditions(non-robust). Furthermore, wavelength tuning of such a system requiresmechanical movement of the tuning elements such as rotation ortranslation of a nonlinear crystal, rotation of cavity mirrors, etc.,which is incompatible with fast wavelength tuning or switching.Therefore, OPOs can not serve as practical ultrashort-pulse sources forproducing multiple-wavelength pulses directly wit mode-locked oscillatoroutput.

SUMMARY OF THE INVENTION

[0011] It is a general object of the present invention to provide amethod and apparatus for generating ultrashort optical pulses at avariable or adjustable optical wavelength from a single source whichprovides ultrashort optical pulses at a fixed optical wavelength.

[0012] It is a further object of the present invention to provide amethod and apparatus for generating ultrashort optical pulses at aplurality of optical wavelengths using a single source which providesultrashort pulses at a fixed optical wavelength Another object of thepresent invention is to provide fast control of the output of a lasersystem in order to select between a plurality of wavelength conversionchannels.

[0013] Still another object of the present invention is to provide aplurality of wavelengths at the single output of a laser system bycombining outputs from separate wavelength-conversion channels into asingle output beam.

[0014] Yet another object of the present invention is to enableefficient multiple-wavelength or adjustable-wavelength operation atrelatively low pulse energies and powers which are compatible withexisting ultrashort-pulse laser oscillators. An additional object of thepresent invention is to implement such a system using components whichare robust, compact and well-suited for large-volume fabrication orderto provide a compact, robust, easily manufacturable and cost-effectiveapparatus.

[0015] It is a further object of the present invention to implement suchmultiple-wavelength laser systems in optical imaging systems, where theability to select from a plurality of optical-signal wavelengths or tosimultaneously use a plurality of optical-signal wavelengths isessential to extend imaging capabilities.

[0016] The aforesaid objects are achieved individual and in combination,and it is not intended that the present invention be construed asrequiring two or more of the objects to be combined unless expresslyrequired by the claims attached hereto.

[0017] In accordance with the present invention, these objects areachieved in a system having a first part comprising a laser system forproducing ultrashort pulses at a fixed wavelength, and a second partcomprising at least one and preferably a plurality ofwavelength-conversion channels. A wavelength-controlling element (orelements) is disposed between the laser generator and thewavelength-conversion channels, which element(s) directs the pulses fromthe laser generator into at least one of the wavelength conversionchannels. Another component or plurality of components is disposeddownstream of the wavelength-conversion channels and serves to combineoutputs from separate wavelength-conversion channels into a singleoutput channel.

[0018] According to the present invention, novel optical waveguidedevices are used for the wavelength-conversion channels,wavelength-control and beam-control elements. Preferably, a fiber lasersystem is used for generating single-wavelength, ultrashort pulses.

[0019] The multiple-wavelength laser system of the present inventionadvantageously replaces a plurality of different, single-wavelengthlaser systems. One particular application for the system is amulti-photon microscope, where the ability to select theultrashort-signal wavelength of the laser source accommodates any singlefluorescent dye or several fluorescent dyes simultaneously.

[0020] Another application for the present invention is in systems thatrequire ultrashort optical pulses at wavelengths that are different fromthe wavelength of the pulse-generating laser. For example, the system ofthe present invention can shift the ultrashort pulse wavelength toapproximately 1.3 μm for optical adherence tomography (OCT), wheretissues are most transparent Similarly, the system of the presentinvention is capable of shifting the wavelength of the ultrashortoptical pulses into a range of wavelengths (1.04 to 1.12 μm) which canbe amplified by Yterbium amplifiers to produce very high powerultrashort pulses for applications such as machining printing anddisplays.

[0021] The above and still farther objects, features and advantages ofthe present invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereto,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

[0022] All of the above-referenced articles are incorporated herein byreference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagrammatic view of an ultrashort-pulse laser sourceaccording to the present invention.

[0024]FIG. 2 is a diagrammatic view of a preferred waveguide structurefor the wavelength conversion channels of the present invention.

[0025]FIG. 3 is a graph illustrating the theoretical optical parametricgeneration (OPG) threshold-energy dependence on the pump pulse (inwaveguide) duration for bulk and waveguide structures inperiodically-poled lithium niobate (PPLN).

[0026]FIG. 4 is a graph illustrating the measured optical parametricgeneration (OPG) conversion efficiency as a function of pump energyaccording to the present invention.

[0027]FIG. 5 is a graph illustrating the measured signal and idlerwavelengths versus pump wavelength at 100° C.

[0028]FIG. 6 is a diagrammatic view of multiple-wavelength output usinga single waveguide in accordance with the present invention.

[0029]FIG. 7 is a diagrammatic view of a multiple-wavelength,ultrashort-pulse generating system according to the present invention.

[0030] FIGS. 8-10 are diagrammatic views of an optical combining switchmatrix, formed in a surface of a substrate, for switching pulses in oneor both of two waveguides into an output waveguide.

[0031]FIG. 11 is a diagrammatic view of an optical combining switchmatrix (OCSM) capable of is combining ultrashort optical pulsestraveling in three wavelength conversion channel waveguides into asingle output waveguide.

[0032]FIG. 12 is a diagrammatic view of an optical split switch matrix(OSSM) for selectively distributing ultrashort pulses into threewavelength conversion channels from one, single-wavelength pulse source.

[0033]FIG. 13 is a diagrammatic view of an optical split switch matrix(OSSM) for selectively distributing ultrashort pulses into treewavelength conversion channels using an acousto-optic device.

[0034]FIG. 14 is a diagrammatic view of an optical split switch matrix(OSSM) for selectively distributing ultrashort pulses into threewavelength conversion channels using an electro-optic device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035]FIG. 1 is a top-level diagram illustrating a system for providingultrashort pulses with an adjustable or variable optical wavelength or aplurality of wavelengths according to the present invention. The systemincludes an ultrashort pulse laser (UPL) 10 for producing ultrashortoptical pulses at a fixed wavelength and at least onewavelength-conversion channel (WCC) 12 ₁-12 _(n).

[0036] UPL 10 is preferably a mode-locked fiber oscillator which canprovide picosecond or ferntosecond optical pulses with typical pulseenergies between 10 pJ (10×10⁻¹²) to 10 nJ (10×10⁻⁹ J) and typicalaverage powers between 0.1 mw to 100 mw. The mode-locked fiberoscillator can have any of a variety of possible designs, such as thosedescribed in the above-referenced articles by Fermann et al. It ispreferable, for the reasons outlined above, that the fiber oscillatorhave an all-fiber cavity without any non-fiber dispersion-controllingelements. Consequently, the preferable operation wavelength is 1.55 μm.

[0037] One important feature of the embodiment illustrated in FIG. 1 isthat the wavelength conversion in the WCCs is obtained in an opticalwaveguide. As explained above, use of nonlinear conversion in'the volumeof curly known nonlinear materials does not allow optical parametricgeneration to be achieved using unamplified output from a mode-lockedfiber laser or, in general from any other existing mode-lockedultrashort-pulse laser. It has been experimentally demonstrated for thefirst time that, by using optical parametric generation in speciallydesigned waveguides in periodically-poled lithium niobate (LiNbO₃), theOPG threshold can be lowered into the energy range accessible withultrashort-pulse oscillators.

[0038] An essential difference between parametric generation in a bulkcrystal and in an optical waveguide is that the latter allows theoptical beam to be confined to a small cross-sectional area and allowsthe optical beam to propagate along the whole waveguide length withoutdiffraction spreading. In contrast, propagation of a free-space beam ina volume of an optical crystal results in diffraction spreading.Consequently, a significantly higher optical intensity over a longpropagation length in an optical waveguide results in significantlyhigher optical parametric gain compared to bulk crystal for the sameoptical pump power.

[0039] Furthermore, maximum interaction on length between two or moreultrashort pulses is limited due to different group-velocities atdifferent optical wavelengths. This maximum, walk-off limited lengthl_(walk-off) is determined by the duration of the pulse Δτ and thegroup-velocity mismatch (GVM) parameter ν_(GVM) of an optical material:l_(walk-off)=Δτ/ν_(GVM).

[0040] Quantitatively, the advantage of OPG in an optical waveguidecompared to the confocally focussed beam in the bulk of the samenonlinear material (at the degeneracy) can be expressed by the followingformula:$\frac{P_{{th} \cdot {{conf}.}}}{P_{{th} \cdot {{waveg}.}}} = \frac{{{\lambda 1}\quad}_{{walk} \cdot {off}}}{2\quad {nA}_{{waveg}.}}$

[0041] Here, P_(th.conf.) and P_(th.waveg.) are threshold peak-powersfor pump pulses in a bulk crystal and in a waveguide, respectively, λand n are signal wavelength and refractive index at the degeneracypoint, and A_(waveg.) is the waveguide cross-sectional area. Higherthreshold peak-power requires higher pump pulse energies. Therefore, theadvantage of using an optical waveguide compared to a bulk crystal isinversely proportional to the pulse duration. Note that, for a bulkmaterial, OPG threshold is independent of the pulse duration.

[0042] As described above, the lowest OPG threshold in bulk crystal hasbeen achieved in a periodically-poled lithium niobate (PPLN). Therefore,the preferable material for a parametric waveguide is PPLN, althoughother periodically-poled ferroelectric optical materials such as PPlithium tantalate, PP KTP, etc. can also be advantageously used. Theoptical waveguides are preferably fabricated in a PPLN substrate usingknown titanium indiffuision (TI) or proton-exchange (PE) (or acombination of titanium indiffusion and proton-exchange (TIPE))techniques.

[0043]FIG. 2 illustrates a preferred waveguide structure for the WCCs ofthe present invention. The optical parametric generation (OPG) stage 14is proceed by a segmented mode-converter structure 16. Themode-converter 16 can have a design similar to that described by Chou etal. in “Adiabatically tapered periodic segmentation of channelwaveguides for mode-size transformation and fundamental modeexcitation”; Optics Letters, Vol. 21, No. 11; June, 1996, incorporatedherein by reference in its entirety. Use of the mode converter 16 isadvantageous, since the OPG waveguide is single-mode at the longer,parametrical signal wavelength (in this particular embodiment, at ˜1.55μm) but is multi-mode at the shorter, pump wavelength (in thisparticular embodiment, at 780 nm). Therefore, it is difficult to excitea single, fundamental mode at the pump wavelength in such a waveguide bya direct coupling of a pump into the waveguide. Best performance, interms of threshold, stability and conversion efficiency can be achievedwhen the pump is coupled into this mode-converter port 16 first, whereit is converted into a fundamental mode, and then launched into the OPGsection 14 in a fundamental transversal mode.

[0044]FIG. 3 illustrates the theoretical OPO threshold-energy dependenceon the pump pulse (in-waveguide) duration for bulk (dotted line) andwaveguide (solid line) structures in PPLN. The OPG threshold energy iscalculated using the above formula and the measured 50 nJ OPGenergy-threshold for bulk PPLN. The experimental energy-threshold valuemeasured for 2 ps long pump pulses is ˜340 pJ, as shown in FIG. 3 by abullet The threshold level constitutes a reduction of approximatelytwo-orders of magnitude for this particular pulse duration and thusdemonstrates that OPO can be achieved with picosecond and subpicosecondpulse durations essentially in the energy range accessible withmode-locked lasers. One example of such a fiber oscillator is given inthe above-mentioned article by Fermann et al. (Electronics Letters, Vol.31, No. 3), providing 6-10 nJ pulses for 2-4 ps duration, which aresufficient to directly drive the waveguide OPG wavelength conversionchannels of the present invention.

[0045] As illustrated graphically in FIG. 4, efficient energy conversioncan be achieved with this 15 structure. Maximum conversion efficienciesof ˜25% have been reached for pump energies at approximately 4-5 timesthe OPG threshold.

[0046] The converted optical wavelength can be adjusted by adjusting thetemperature of the waveguide (i.e., by controlling the temperate ofsubstrates in which the waveguide is formed) thus accessing a pluralityof optical wavelengths with a single waveguide. The OPG 14 is capable ofsimultaneously producing two different optical wavelengths, the shorterof which is called “signal” and the longer of which is called “idler”.Therefore, a single WCC is suitable for generating two required opticalwavelengths by properly choosing the pump wavelength and theperiodic-poling period such as to satisfy energy-conservation andmomentum conservation laws for all three (pump, signal and idler)optical wavelengths. As an example, FIG. 5 illustrates the measuredsignal and idler wavelengths versus pump wavelength at temperature of100° C. and a quasi-phase-matched (QPM) grating period of 15 μm.

[0047] Furthermore, multiple-wavelengths can be accessed with a singlechip containing a plurality of waveguides with differentelectrically-poled periods, as shown in FIG. 1. Each predesignedwavelength can be accessed by translating the crystal in the transversaldirection to select the required waveguide.

[0048] In accordance with the present invention, each WCC optionallyincludes at least one harmonic generator HG 18 before the OPG stage 14and at least one harmonic generator HG 20 at the PG stage 14. Generally,this allows generation of optical wavelengths shorter than themode-locked laser wavelength. All waveguides can be manufactured on asingle chip thus simplifying the system and eliminating additionalin-waveguide coupling losses. If a free-space pump is first coupled intoa waveguide harmonic-generator which is a single-mode at this initialwavelength, e.g., at 1550 nm, then the wavelength-converted beam at ashorter wavelength, in general, will be obtained in a fundamental modeand can be directly launched into the OPG stage (on the same chip). Modeconverters then may not be necessary.

[0049] An example of a configuration with a multiple-wavelength outputusing a single waveguide as a wavelength conversion channel is shown inFIG. 6. For example, for two photon microscopy, a specific set ofultrashort-pulse wavelengths at ˜680 nm, ˜780 nm and ˜915 nm is highlydesirable. This can be accomplished by launching a ˜1550 nm fiber-laserinput into a waveguide, the first section of which constitutes asecond-harmonic generator 60, implemented through the correct PPLNperiod (which should be designed Sing into account the exact geometry ofthe waveguide) and the temperature of the waveguide substrate. Thewaveguide can have the same width throughout all sections, provided thatthe 1.55 μm input is single-mode. Then, the generated second-harmonic isin the fundamental mode. The doubled output of the fiber laser at ˜780nm is further transmitted into the OPO section 62 of the waveguide tosimultaneously generate ˜1360 nm as the signal wavelength, and ˜1830 nmas the idler wavelength. The specified wavelengths can be obtainedthrough a certain PPLN period used for the OPG section, according thefactors described above. These two generated signal and idlerwavelengths can be separately doubled in further sections of thewaveguide to provide 680 nm and 915 nm wavelength pulses, respectively.The remaining 780 n pump is transmitted together with these twowavelengths to the output to be used, for example, for two-photonmicroscopy. The final stages of the device, containing harmonicgenerators 64 and 66 for OPG output can be implemented on the samesubstrate, separately on a different substrate or substrates or evenusing bulk material.

[0050] A general embodiment of a multiple-wavelength, ultrashort-pulsegenerating system according to the present invention is shown in FIG. 7.The system for generating and controlling a multiple-wavelengthultrashort-pulse output includes an ultrashort pulse laser (UPL) 10 forproducing ultrashort optical pulses at a fixed wavelength an opticalultrashort-pulse amplifier (UPA) 22 for increasing power and energy ofthe ultrashort pulses from UPL 10, an optical split switch matrix (OSSM)24 for distributing ultrashort pulses into a plurality ofwavelength-conversion channels, at least one and preferably a pluralityof wavelength-conversion channels (WCCs) 12 ₁ to 12 _(n), each of whichincludes a parametric-generation stage (PG) 14 and optional harmonicgeneration (HG) stages 18 and 20, and an optical combining switch matrixOCSM 26 at the output of the system to combine output ports of aplurality of WCCs to provide a single output beam (the OSSM and OCSM arenot necessary if only one WCC is present).

[0051] If the pulse energy directly produced by a mode-locked fiberoscillator is insufficient for driving the waveguide-WCCs, the laseroutput can be amplified in an ultrashort-pulse amplifier UPA 22.Preferably, such amplifier is a fiber amplifier. Very importantly, thelow energy required to operate the waveguide-WCCs allows one to use arelatively simple fiber amplifier design Pulses in the 1-10 nJ range andhigher can be obtained either directly or by using compact and simplechirped-pulse amplification schemes based on chirped fiber gratings orchirped-period poled lithium niobate compressors C-PPLN, as described byGalvanauska et al. in “Use of Chirped-Period-Poled Lithium Niobate forChirped Pulse Amplification in Optics Fibers”; Ultrafast Optics '97,Montery Calif.; August, 1997, incorporated herein by reference in itsentirety.

[0052] The optical combining switch matrix (OCSM) 26 is capable ofselecting a particular laser source from a plurality of WCCs. Aconceptual plan view of a basic OCSM formed in a surface of aferroelectric (e.g., PPLN) substrate is illustrated in FIG. 8. The basicOCSM is capable of switching pulses of wavelength λ₁ from waveguide 30and/or pulses of wavelength λ₂ from waveguide 32 into a main trunk(output) waveguide 34. As explained above, the substrate is preferablymade of a ferroelectric material such as lithium niobate or lithiumtantalate. The optical waveguides are fabricated using titaniumindiffusion (TI) or proton-exchange (PE) or a combination of titaniumindiffusion and proton-exchange (TIPE). The optical switches arefabricated by bringing certain regions of the two optical waveguidessufficiently close together that the laser light can be switch from onewaveguide to another.

[0053] As shown in FIG. 9, in the absence of an external electric field,ultrashort pulses in waveguides 30 and 32 will not be switched to themain trunk waveguide 34 and will continue to propagate in waveguides 30and 32 to optically terminated ports. The application of specificelectrical voltages will cause the complete coupling of the ultrashortpulses in waveguide 30 and/or waveguide 32 into the main trunk waveguide34. For example, as shown in FIG. 10, the ultrashort light pulses inwaveguide 32 are coupled into the main trunk waveguide 34 by applicationof voltage V2 across the gap between the two waveguides. Opticaldirectional couplers 36 and 38, such as those described in “Introductionto Optical Electronics”, Anon Yariv, pp. 391-395, Holt, Rinehart andWinston, 1976, incorporated herein by reference in its entirety can beused to apply the respective electric fields between waveguides 30 and32 and main trunk waveguide 34. The main trunk waveguide 34 ispreferably fabricated using only the TIPE process. This allows thevarious ultrashort pulses to propagate relatively efficiently throughthe main trunk waveguide 34 and hence will provide a common port for altthe WCCS.

[0054]FIG. 11 is a diagrammatic plan view of the OCSM 26 shown in FIG.7. FIG. 11 shows an OCSM capable of handling three WCCs, although theOCSM can be designed to handle any number of WCCs in accordance with theprinciples illustrated in FIG. 11. The OCSM 26 comprises three opticaldirectional couplers 40, 42 and 44 fabricated in a ferroelectricmaterial. The main optical waveguides 48, 52 and 56 of three WCCs arefind using T he waveguide 48 of a first WCC propagating 500 nmultrashort pulses forms the center input waveguide with a refractiveindex of n1. In the off state (zero applied voltage), the 500 nmultrashort pulses continue to propagate in waveguide 48 (which becomethe center portion of output waveguide 46). By application of a voltageV1, the 500 nm ultrashort pulses are switched to a laser dump port 50and absorbed. Within the output waveguide 46, the 500 nm pulses tend topropagate primarily within the region having the n1 refractive index,thereby preserving a high degree of single mode operation.

[0055] A second WCC waveguide 52 providing 780 nm wavelength pulses iscoupled to the output waveguide 46 by the second hybrid opticaldirectional coupler 42. In the off state, the 780 nm ultrashort pulsesare dumped into an optically terminated port 54. By application of anelectrical voltage V2, the 780 nm ultrashort pulses can be switched tothe output waveguide 46 with the hybrid TIPE waveguide of refractiveindex n2. The 780 nm pulses propagate primarily within the portion ofthe output waveguide having n1 and n2 indices of refraction, therebypreserving a high degree of single mode operation (i.e., the combinedcross-sectional area of the n1 and n2 regions is consistent with singlemode propagation for 780 nm ultrashort pulses).

[0056] The OCSM 26 further comprises another hybrid optical directionalcoupler 44 and an additional TIPE waveguide section of the outputwaveguide 46 with refractive index n3, where n1>n2>n3. The role of thisadditional TIPE waveguide section is to enable the propagation of the980 nm wavelength pulses within the output waveguide 46. Specifically,the 980 nm pulses propagate primarily across the n1, n2 and n3 regions,where the combined cross-sectional area of the n1, n2 and n3 regions isconsistent with single mode propagation for 980 nm ultrashort pulses).If zero voltage is applied to the third directional coupler 44, the 980nm ultrashort pulses propagating in waveguide 56 are dumped in thetermination port 58. By applying an electrical voltage of V3 to thehybrid optical directional coupler, the 980 nm wavelength pulses areguided in the common hybrid output port 46.

[0057] Although, for convenience, the output waveguide 46 is shown inFIG. 11 as having separate regions of n1, n2 and n3 refractive indices,it will be understood that the reflective index changes gradually overthe Width of the output waveguide 46, i.e., there is no refractive index“step” between regions n1 and n2 and between regions n2 and n3. Further,it will be understood that the n1, n2 and n3 regions are side-by-side inthe substrate, e.g., the two n2 regions need not be a single regionextending below and around the n1 region.

[0058] The optical directional couplers are preferably hybrid, becausethe TIPE waveguide technology is used here. The use of TIPE waveguidesassists in the ability to combine all three wavelength sources to exitfrom the substrate through a common port and all wavelengths can remainin quasi-single mode operation. As can be see significant complicationis added to the device so that the multiple wavelengths can propagatedown a single waveguide and still be single mode. If the wavelengths areclose enough together, a single waveguide will be single mode for each.

[0059] The OCSM 26 shown in FIG. 11 can be extend to combine any numberof WCCs limited only by the size of the substrate material. With theavailability of 4″ lithium niobate wafer mat it is possible to combineup to ten different WCCL The design of the various TIPE waveguidesections is more critical with greater numbers of WCCs.

[0060] The above description relates to combining of signal pulsesreceived in each of the WCCs. The same principle can be extended toswitching of the idler signal in each of the WCCs as well.

[0061] It should be noted that as an alternative to the above-describednovel OSCM 26, the combining function can be performed usingconventional devices which are external to the integrated optical chip.For example, there are a number of known means for combining multiplewavelengths into a common path. These means have been used in WDMsystems. The simplest means is a series of dichroic mirrors. Anotherapproach is to use a fiber WDM. In general, the OSCM 26 of the presentinventions can employ any method used in WDM systems for combiningdifferent wavelengths.

[0062] The structure of optical split switch matrix (OSSM) 24 of FIG. 7is shown generically in FIG. 12. The OSSM 24 directly feeds ultrashortpulses (e.g., 1.55 nm) from UPL 10 into any one or several of the WCCs.The control of the ultrashort pulses from the input port of the OSSM 24to any of the WCCs is accomplished using either an electro-optic or theacousto-optic method, as described hereinbelow.

[0063]FIG. 12 illustrates the use of 1×3 optical directional couplers 60to distribute the input radiation to any or all of the output ports. The1.55 μm wavelength pulses are fed into an optical waveguide fabricatedby TI or PE or TIPE on a ferroelectric substrate such as lithium niobateor lithium tantalate. All the waveguides have the same widthcross-sectional area which is designed for single-mode propagation atthe source wavelength The condition of the splitting action is governedby applying voltages V1 or V2 to the 1×3 optical directional couplers60. Appropriate mode converters 16 can used in WCCs to ensure optimizeddevice operations, i.e., minimum excess loss and high interactionefficiency in the WCCs (see FIG. 2). The switching voltage applied tothe 1×3 OSSM can be synchronized to the switching of the OCSM 26described above. The OSSM 24 shown in FIG. 12 can be realized usingelectro-acoustic or electro-optical active switches. It is also possibleto use guided-wave optical gratings to realize the above OSSM.

[0064]FIG. 13 illustrates a novel implementation of a 1×3 OSSM based onsurface acoustic waves (SAWs) generated by the interdigital transducersIDT1 70 and IDT2 72. As shown in FIG. 13, IDT1 70 and IDT2 72 aredisposed on the substrate surface. The optical waveguide regions markedwith Δn1, Δn2, and Δn3 are of slightly higher index an the base 1×3optical waveguide structure. The substrate material is preferablyferroelectric with the waveguides being fabricated using TI. Theslightly higher index waveguide regions are fabricated using PE. Withproper annealing, the refractive index change can be minimized, asrequired by this configuration. When no electrical signals are appliedto the interdigital electrode transducers, the path of the 1.55 μm laserlight will propagate straight into the middle output port of the 1×3OSSM. When a voltage V1 is applied to IDT1 70, the generated SAWs willdeflect the 1.55 μm laser light into the first (e.g., upper) output portof the 1×3 structure. In the same manner, if a voltage V2 is applied tothe IDT2 72, the 1.55 μm ultrashort pulse is deflected into the third(e.g., lower) output port of the 1×3 waveguide structure. The directionand amount of deflection of the input pulse depends both on the appliedvoltages and the Δn values. Placement of the IDTs on the surface of thesubstrate can be optimized to improve efficiency. Efficiencies greaterthan 90% can be realized with such a configuration. The insertion lossis minimized by the presence of the Δn structure.

[0065] For the equal distribution of the input laser radiation into allthree output ports of the 1×3 OSSM, the Δn's of the tree hybridwaveguide regions can be increased by shorter anneal times or longer PEtimes. In this mode of option, both of the applied voltages V1 and V2 tothe OSSM are required to optimize the equal splitting action.

[0066] Instead of using acousto-optic element for deflection of theinput pulses, the switching action can be implemented using anelectro-optic induced grating (EOG) by using a pair of grating metallicelectrodes on the ferroelectric substrates. FIG. 14 shows a novelimplementation of such a 1×3 OSSM. The 1×3 optical waveguide device andthe presence of the three appropriate higher Δn regions are similar tothose described in the acousto-optic based 1'3 OSSM shown in FIG. 13. Byapplying a voltage V1 to the EOG1 80, a periodic refractive index changeis induced similar to that generated by an IDT in FIG. 13. The period ofthe metallic electro-optic induced grating structure is designed suchthat the 1.55 μm input pulse is switched to the first (e.g., upper)output port of the 1×3 waveguide device. If no voltage is applied the1.55 μm input pulse will go directly into the middle port of the 1×3OSSM device. If a voltage V2 is applied to the EOG2 82, the incoming1.55 μm input pulse is switched appropriately to the third (e.g., lower)output port of the 1×3 device. The refractive index changes of the threehybrid waveguide struck in the 1×3 OSSM can be increased to allow forequal splitting of the incoming pump laser radiation into the threeoutput ports. Both of the EOGs 80 and 82 are then used to optimize thesplitting. The acousto-optic and electro-optic devices described herecan be, for example, those used for switching in telecommunicationcircuits. Of course, other means of switching for integrated opticalcircuits for telecommunication applications can also be used.

[0067] As with the OCSM described above, the OSSM can be extended from a1×3 to a 1×10 structure. Again, the critical limitation is the size ofthe ferroelectric wafer. The hybrid PE sections in the larger than 1×3OSSM element can be realized by multiple PE processes to compensate forhigher splitting losses at larger angular splits, as required by theoverall OSSM, WCC and OCSM configuration. The OSSM 24, WCCs and OCSM 26are preferably formed on a single substrate.

[0068] The specific configuration for the multi-wavelength source of thepresent invention is very much dependent on the application With amulti-wavelength source in a system, one has a system with much expandedcapability. In accordance with one preferred embodiment, the multi.wavelength source of the present invention is used as a source for atwo-photon microscope. The purpose of the laser is to allow a largernumber of dyes which require different excitation wavelengths to be usedin one system This expands the usefulness of such microscopes. Forexample, it may be useful to have We laser quickly tuned from 780 nm, to700 nm and to 850 nm. Similarly, it may be useful to have the laserquickly tuned or simultaneously generate pulses at wavelengths of 680nm, 780 nm and 915 nm (see FIG. 6). Such a laser can be used for thedyes which are favorably excited by each of these wavelengths.

[0069] For example, dyes rhodamine, H729 and HOE33342 are excited bytwo-photon excitation at 780 nm. These are used for labelinghepatocytes, colon carcinoma and nuclei, respectively. Dyes Fura-2,Indo-1, Green Fluorescent Protein and FITC are excited by two-photonexcitation at 700 nm. These are most useful for labeling structures andtracking calcium transduction in dendritic neural structures. Threephoton excitation at roughly 280 nm using 850 nm from the laser can beused to cause serotonin, tryptophan and NADH or NAD(P)H toautofluroresce. Serotonin is a key gauge of neural activity as the chiefaminergic chemical in the brain. NADH and NAD(P)H are used to tackactivity used in identifying, for instance, skin melanoma.

[0070] Since a multi-wavelength source is very desirable forconventional confocal microscopy as well, the source of the presentintention can be used for conventional rather than multiphoton isexcitation by using a dye with absorption at the fundamental wavelengthrather than one half the wavelength. It may be desirable to disperse thetemporal pulse to a longer pulse so the peak power is not sufficient fortwo-photon excitation Therefore, in this case, the laser must be capableof switching to the common wavelengths used in confocal microscopy, suchas 482 nm and 514 nm of argon ion lasers, 632 nm of HeNe and 780 nm ofTi:sapphire.

[0071] It is also very desirable to increase the power of ultrafastsources. Higher powers are needed for many applications such as thoserelated to machining. It has been demonstrated that ultrafast fiberlasers can be amplified to one watt with erbium amplifiers. Presently,however, power is limited to about 10 watts, which is too low for manymachining applications. Recently, Yterbium fiber amplifiers have beendemonstrated to yield 40 watts output These amplifiers are moreefficient than erbium and are preferred for higher powers. These fibershave a large bandwidth and can support a very short pulse, but there isnot a commercial ultrafast source at these wavelengths (i.e., 1.04-1.12μm). With an OPO frequency converter, the output of a more conventional(e.g., Erbium) ultrafast source can be converted to the wavelengthsupstream of the Yterbium amplifier. This allows for a very high powersource.

[0072] Another high power amplifier which can reach even higher powersis Yterbiwn YAG. Yterbium YAG has been shown to give 200 watts ofaverage power and again there is not a conventional ultrafast source atthe wavelength of Yterbium YAG.

[0073] One of the main applications for the Yterbimn amplifier is as anRGB source for commercial display or printing purposes. Again, afteramplification in the Yterbium amplifier, an OPG waveguide device can beadded which can simultaneously or separately convert the ultrafastpulses to red, green, blue wavelengths. The integrated optics circuit ofthe type described above could also include this switching circuit toturn the colors on or off for the image formation Thc ultrafast pulseshave the advantage in that the efficient conversion obtained with highpeak-power and large bandwidth at each color minimizes the speckleobtained from the laser (speckle makes the image appear grainy to theeye).

[0074] Optical coherence tomography (OCT) is being developed as amedical and ophthalmic imaging tool. It is capable of using light toimage through human tissue which scatters light sly. OCT has beendemonstrated to give images with better resolution than other medicalimaging techniques such as MRI, computerized tomography, or ultrasound.Axial resolution is 10 microns, and can be reduced to 2 microns whenusing a short coherence length light source such as a ferntosecondlaser. However, the depth of imaging is limited to about 3 mm. Onedesirable feature of OCT is that it can use a simple and cheap lightsource such as a superluminscent laser diode. However, betterperformance is obtained using a mode-locked laser. For example, in invivo imaging of the heart of a frog embryo, it takes 20 seconds toacquire an image when using a superluminescent diode, but only 0.25seconds when using a mode-locked laser, allowing researchers to capturethe motion of the beating heart during diastolic and systolic phases.Rapid scanning (2000 Hz) can be employed to achieve this fast imageacquisition. Both mode-locked Ti:sapphire and mode-locked Cr:Forsteritehave been used for OCT. Cr:Forsterite is especially well suited forimaging in biological tissues because of its wavelength (1300 nm);scattering effects that limit imaging depth are reduced at longerwavelengths. Because the method is compatible with fiber technology, ithas been successfully used for endoscopy. As reported by Tearney et al.in “Rapid acquisition of in vivo biological images by use of opticalcoherence tomography”; Optics Letters, Vol. 21, No. 17; September 1995,incorporated herein by reference in its entirety, a radially-scanningcatheter-endoscope probe with rapid image acquisition has beendemonstrated OCT has been demonstrated in a number of clinical andresearch trials including: cancer detection in the human stomach wall;subsurface imaging and histology of the porcine esophagus wall;performing optical biopsy to replace excisional biopsy; and mappingblood flow velocities using Color Doppler OCT (CDOCT). Coupled withcatheter, endoscopic, or laparoscopic delivery, OCT holds the promise ofenabling the screening and diagnosis of a wide range of diseasesincluding cancerous and precancerous tissue changes without the need forexcisional biopsy and histological processing. In conjunction withconventional microscopy, OCT enables the imaging of internal structuresin living specimens without the need for sacrifice and histology.

[0075] Therefore, for the purposes of OCT imaging in human tissue, asource lasing at 1.3 microns is desired. An erbium-doped fiber laserwhich is converted with an OPG waveguide device to 1.3 μm would besuitable for this application.

[0076] Having described preferred embodiments of a new and improvedmethod and apparatus for displaying question and answer data on pluraldisplays, it is believed that other modifications, variations andchanges will be suggested to those skilled in the art in view of theteachings set forth herein. It is therefore to be understood that allsuch variations, modifications and changes are believed to fall withinthe scope of the present invention as defined by the appended

What is claimed is:
 1. An ultrashort pulse source for generatingultrashort optical pulses at a plurality of different wavelengths,comprising: a mode-locked laser generating ultrashort optical pulses; awavelength conversion channel for converting a wavelength of saidultrashort optical pulses to a different wavelength, comprising anoptical waveguide formed in a substrate, said optical waveguideincluding an optical parametric generation portion for parametricallyamplifying said ultrashort optical pulses.
 2. The ultrashort pulsesource according to claim 1, wherein said substrate comprises aperiodically-poled ferroelectric optical material.
 3. The ultrashortpulse source according to claim 2, wherein said periodically-poledferroelectric optical material is one of: lithium niobate, lithiumtantalate, and KTP.
 4. The ultrashort pulse source according to claim 1,wherein said wavelength conversion channel converts the wavelength ofsaid ultrashort optical pulses as a function of at least one of: atemperature of the wavelength conversion channel; a wavelength of lightpumped into said wavelength conversion channel; and a periodic-polingperiod of an electric field in said wavelength conversion channel. 5.The ultrashort pulse source according to claim 1, wherein saidmode-locked laser is a fiber laser.
 6. The ultrashort pulse sourceaccording to claim 1, wherein said fiber laser is an erbium-doped fiberlaser.
 7. The ultrashort pulse source according to claim 1, wherein saidmode-locked laser is a mode-locked Ti:sapphire laser or a mode-lockedCr:Forsterite laser.
 8. The ultrashort pulse source according to claim1, wherein said wavelength conversion channel further comprises at leastone harmonic generator for generating ultrashort optical pulses whosewavelength is shorter than the wavelength of the ultrashort opticalpulses generated by said mode-locked laser.
 9. The ultrashort pulsesource according to claim 1, further comprising an ultrashort-pulseamplifier upstream of said wavelength conversion channel for amplifyingsaid ultrashort optical pulses.
 10. The ultrashort pulse sourceaccording to claim 9, wherein said ultrashort-pulse amplifier is anerbium fiber amplifier.
 11. The ultrashort pulse source according toclaim 1, further comprising an ultrashort-pulse amplifier downstream ofsaid wavelength conversion channel, wherein said ultrashort-pulseamplifier is one of: a Yterbium amplifier; and a Yterbium YAG amplifier.12. The ultrashort pulse source according to claim 1, wherein saidwavelength conversion channel is one of a plurality of wavelengthconversion channels each of which comprises an optical waveguide,converts the wavelength of said ultrashort optical pulses, and causesparametric amplification of said ultrashort optical pulses, saidultrashort pulse source fiber comprising: a first optical switch fordirecting the energy of said ultrashort optical pulses into at least oneof said wavelength conversion channels; and a second optical switch fordirecting ultrashort optical pulses in each of said wavelengthconversion channels into a single output waveguide.
 13. The ultrashortpulse source according to claim 12, wherein said first optical switchcomprises: a waveguide, having a first refractive index, connecting asingle input optical waveguide to n output optical waveguide, where n isan integer greater than 1, said waveguide including n regions ofdifferent refractive index for guiding optical pulses from said singleinput optical waveguide to respective ones of said n output opticalwaveguides.
 14. The ultrashort pulse source according to claim 13,wherein said waveguide is formed in a ferroelectric optical material bytitanium indiffusion and said n regions of different refractive indexare formed by proton exchange.
 15. The ultrashort pulse source accordingto claim 13, wherein the energy of ultrashort optical pulses receivedfrom said single input optical waveguide is distributed substantiallyequally among a plurality of said n output optical waveguides.
 16. Theultrashort pulse source according to claim 13, wherein said firstoptical switch further comprises a 1×n directional coupler for directingthe energy of ultrashort optical pulses received from said single inputoptical waveguide into any single one or any combination of said nregions of different refractive index, whereby the energy of theultrashort optical pulses received from said single input optical waveguide is guided into any single one or any combination of said n outputoptical waveguides.
 17. The ultrashort pulse source according to claim16, wherein said 1×n directional coupler comprises n acousto-opticdevices for generating surface acoustic waves capable of deflectingultrashort optical pulses received at said single input opticalwaveguide into respective ones of said n output optical waveguides. 18.The ultrashort pulse source according to claim 17, wherein said nacousto-optic devices are interdigital transducers.
 19. The ultrashortpulse source according to claim 16, wherein said 1×n directional couplercomprises n electro-optic devices capable of deflecting ultrashortoptical pulses received at said single input optical waveguide intorespective ones of said n output optical waveguides.
 20. The ultrashortpulse source according to claim 19, wherein said n electro-optic devicesare electro-optic induced gratings.
 21. The ultrashort pulse sourceaccording to claim 12, wherein said second optical switch comprises: anoutput waveguide; and n optical directional couplers, where n is aninteger greater than 1, each of which respectively couples one of noptical waveguides to said output waveguide, wherein application of avoltage to one of said n optical directional couples couplers ultrashortoptical pulses propagating in a corresponding one of said n opticalwaveguides into said output waveguide.
 22. The ultrashort pulse sourceaccording to claim 21, wherein ultrashort optical pulses from aplurality of said n optical waveguides propagate in said outputwaveguide simultaneously.
 23. The ultrashort pulse source according toclaim 21, wherein said output waveguide comprises n axially extendingportions each having a different refractive index, wherein ultrashortoptical pulses coupled into said output waveguide from an opticalwaveguide i propagate substantially in axially extending portions 1through i, where i is an integer from 1 to n, such that ultrashortoptical pulses of different wavelengths propagate in said outputwaveguide substantially in a single mode.
 24. The ultrashort pulsesource according to claim 21, wherein said output waveguide is formed ina ferroelectric optical material by at least one of titanium indiffusionand proton exchange.
 25. An optical switch, comprising: a waveguide,having a first refractive index, connecting a single input opticalwaveguide to n output optical waveguides, where n is an integer greaterthan 1, said waveguide including n regions of different refractive indexfor guiding optical pulses from said single input optical waveguide torespective ones of said n output optical waveguides; and a 1×ndirectional coupler for directing the energy of an ultrashort opticalpulse propagating in said single input optical waveguide into any singleone or any combination of said n regions of different refractive index,whereby the energy of the ultrashort optical pulse is guided into anysingle one or any combination of said n output optical waveguides. 26.The optical switch according to claim 25, wherein said 1×n directionalcoupler comprises n acousto-optic devices for generating surfaceacoustic waves capable of deflecting the ultrashort optical pulse intorespective ones of said n output optical waveguides.
 27. The opticalswitch according to claim 26, wherein said n acousto-optic devices areinterdigital transducers.
 28. The optical switch according to claim 25,wherein said 1×n directional coupler comprises n electro-optic devicescapable of deflecting the ultrashort optical pulse into respective onesof said n output optical waveguides.
 29. The optical switch according toclaim 28, wherein said n electro-optic devices are electro-optic inducedgratings.
 30. The optical switch according to claim 25, wherein theenergy of said ultrashort optical pulse is distributed substantiallyequally among a plurality of said n output optical waveguides.
 31. Theoptical switch according to claim 25, wherein said waveguide is formedin a ferroelectric optical material by titanium indiffusion and said nregions of different refractive index are formed by proton exchange. 32.An optical switch, comprising: an output optical waveguide; and noptical directional couplers, where n is an integer greater than 1, eachof which respectively couples one of n optical waveguides to said outputwaveguide, wherein application of a voltage to one of said n opticaldirectional couplers couples ultrashort optical pulses propagating in acorresponding one of said n optical waveguides into said outputwaveguide.
 33. The optical switch according to claim 32, whereinultrashort optical pulses from a plurality of said n optical waveguidespropagate in said output waveguide simultaneously.
 34. The opticalswitch according to claim 32, wherein said output waveguide comprises naxially extending portions each having a different refractive index,wherein ultrashort optical pulses coupled into said output waveguidefrom an optical waveguide i propagate substantially in axially extendingportions 1 through i, where i is an integer from 1 to n, such thatultrashort optical pulses of different wavelengths propagate in saidoutput waveguide substantially in a single mode.
 35. The optical switchaccording to claim 32, wherein said output waveguide is formed in aferroelectric optical material by at least one of titanium indiffusionand proton exchange.
 36. In combination: a microscope for detectingfluorescence of dyes excited by absorption of ultrashort optical pulses;and an ultrashort-pulse source for supplying ultrashort optical pulsesto said microscope, said ultrashort-pulse source comprising: a laser forgenerating ultrashort optical pulses at a single wavelength; a pluralityof wavelength conversion channels for converting said ultrashort opticalpulses to a plurality of respective different wavelengths; and anoptical switch for switching ultrashort optical pulses from any one orany combination of said wavelength conversion channels into a singleoutput channel, whereby said ultrashort-pulse source is capable ofsupplying ultrashort optical pulses of a plurality of differentwavelengths to said microscope.
 37. The combination according to claim36, wherein microscope excites said dyes using two-photon microscopy.38. The combination according to claim 36, wherein said laser is amode-locked laser.
 39. The combination according to claim 36, whereineach of said wavelength conversion channels includes an opticalparametric generation portion which parametrically amplifies theultrashort optical pulses.
 40. The combination according to claim 39,wherein each of said wavelength conversion channels comprises awaveguide formed in a ferroelectric optical material.
 41. Incombination: a color display for displaying red, green and blue images;and an ultrashort-pulse source for supplying image data to said colordisplay in the form of red, green and blue ultrashort optical pulses,said ultrashort-pulse source comprising: a laser for generatingultrashort optical pulses at a single wavelength; a first wavelengthconversion channel for converting the wavelength of said ultrashortoptical pulses to produce red ultrashort optical pulses; a secondwavelength conversion channel for converting the wavelength of saidultrashort optical pulses to produce blue ultrashort optical pulses; athird wavelength conversion channel for converting the wavelength ofsaid ultrashort optical pulses to produce green ultrashort opticalpulses.
 42. The combination according to claim 41, wherein said laser isa mode-locked laser.
 43. The combination according to claim 41, whereineach of said first second and third wavelength conversion channelsincludes an optical parametric generation portion which parametricallyamplifies the ultrashort optical pulses.
 44. The combination accordingto claim 43, wherein each of said first second and third wavelengthconversion channels comprises a waveguide formed in a ferroelectricoptical material.
 45. An ultrashort pulse source for generatingultrashort optical pulses at a plurality of different wavelengths,comprising: a mode-locked laser generating ultrashort optical pulses;and a wavelength conversion section including an optical parametricgeneration portion; and including a waveguide-based optical switchingsection connecting a single input optical waveguide to n output opticalwaveguides, where n is an integer greater than 1, includingacousto-optical or electro-optical mean for effecting switching ofoptical pulses from said single input optical waveguide to respectiveones of said n output optical waveguides.
 46. The ultrashort pulsesource according to claim 45, wherein said wavelength conversion sectionis formed in a ferroelectric optical material.
 47. The ultrashort pulsesource according to claim 46, wherein said wavelength conversion sectionconverts the wavelength of said ultrashort optical pulses as a functionof at least a periodic-poling period of said ferroelectric material. 48.A multi-wavelength ultrashort-pulse source, comprising: a laser forgenerating ultrashort optical pulses at a single wavelength; a pluralityof wavelength conversion channels for converting said ultrashort opticalpulses to a plurality of respective different wavelengths; and anoptical switch for switching ultrashort optical pulses from any one orany combination of said wavelength conversion channels into a singleoutput channel.
 49. The multi-wavelength ultrashort-pulse sourceaccording to claim 48, wherein each of said wavelength conversionchannels includes an optical parametric generation portion whichparametrically amplifies the ultrashort optical pules.